A murine breast cancer model has been developed for measuring the ability of targeted effector cells to eradicate primary and transplanted mammary tumors. Bispecific antibodies containing anti-CD3 cross linked to antibodies against gp52 from the mouse mammary tumor virus bind specifically to spontaneous or cultured mammary tumor cells, and induce murine T cells to lyse such cells in vitro and block the growth of subcutaneous tumor transplants in vivo. A genetically engineered bispecific F(ab')2 construct has been produced that has the same in vitro targeting activity as conventionally prepared bispecific antibodies. By using anti-CD44 containing bispecific antibodies, we have found that CD44 is a cytotoxic triggering molecule on a subset of human PBL. Cells mediating such targeted lysis reside in the LGL population, are CD56+, and are activated by IL-2.